1. Field of the Invention
The present invention relates to a radar apparatus capable of obtaining the correct azimuth angle of a target object, by means of super-resolution angle measuring processing.
2. Description of the Related Art
For example, in Japanese Patent Application Laid-Open No. 2008-39718, there is disclosed a method in which, after the relative speed of a target object and the distance between the reference point and the target object are calculated by utilizing a FMCW method in which frequency modulation is applied to a continuous wave and based on the beat frequency between a transmission signal and a reflection signal from the target object at a time when the frequency of the transmission signal increases and the beat frequency between the transmission signal and the reflection signal from the target object at a time when the frequency of the transmission signal decreases, the azimuth angle of the target object is obtained by means of super-resolution angle measuring processing such as “MUSIC” (Multiple Signal Classification). “MUSIC” is described, for example, in “Multiple Emitter Location and Signal Parameter Estimation”, by R. O. Schmidt, IEEE Trans. AP-34, 3, pp. 276 to 280 (1986).
The input in super-resolution angle measuring processing such as MUSIC is the vectors of input signals from a plurality of reception antennas. In a radar apparatus, utilizing a FMCW method, disclosed in Japanese Patent Application Laid-Open No. 2008-39718, in order to obtain input-signal vectors, a frequency analysis means based on a fast Fourier transformation (FFT) is utilized in many cases. In a FMCW method, the relative speed of a target object and the distance between the reference point and the target object are calculated, based on the sum of and the difference between the frequency of the beat signal at a time when the frequency increased (referred to as UP chirping, hereinafter) and the frequency of the beat signal at a time when the frequency decreased (referred to as DOWN chirping, hereinafter). Next, in order to obtain the azimuth angle, by utilizing, as input-signal vectors, complex frequency components of the beat signal, at each of the reception antennas upon UP chirping and DOWN chirping, which corresponds to the relative speed of a target object and the distance between the reference point and the target object, one or more azimuth angles can accurately be obtained by means of super-resolution angle measuring processing.
In the conventional radar apparatus, in the case where a plurality of target object exists, as represented in FIG. 3, one of beat frequencies upon UP chirping and beat frequencies upon DOWN chirping may coincide with each other (one another) in some cases. At this moment, the input-signal vector for the super-resolution angle measuring processing also includes the vector of a reception signal from another target object. If the super-resolution angle measuring processing is implemented based on the input-signal vector, not only a correct azimuth angle but also an azimuth angle of another target object is obtained. Accordingly, as represented in FIG. 4, not only the correct relative speed and azimuth angle (θa, in FIG. 4) of the target object and the correct distance between the reference point and the target object, but also the incorrect relative speed and azimuth angle (θb, in FIG. 4) of another target object and the incorrect distance between the reference point and another target object are obtained. In addition, in FIG. 3, reference numeral 20 denotes a radar apparatus; reference numeral 21 denotes a detection target A having a distance of ra from the radar apparatus 20, a relative speed of va, and an azimuth angle of θa; reference numeral 22 denotes a detection target B having a distance of rb from the radar apparatus 20, a relative speed of vb (=0), and an azimuth angle of θb; the abscissa and the ordinate of each of the graphs denote the beat-signal frequency after FFT processing and the power, respectively.